Telomere homeostasis regulates both chromosomal stability and cell replicative lifespan in mammalian cells. As a consequence, alterations in telomere biology play critical roles in the pathogenesis of human cancer and may contribute to aging. Indeed, studies in both human and animal models indicate that telomeres and telomerase, the reverse transcriptase that maintains telomere structure, serve dual roles in oncogenesis, acting both to suppress and facilitate neoplastic transformation. Moreover, mutations of telomerase components required for catalytic activity, TERT and TERC, are found in diseases characterized by stem cell failure including dyskeratosis congenita, subsets of aplastic anemia and idiopathic pulmonary fibrosis. Taken together, these telomerase plays a key role in maintaining telomeres, accumulating evidence suggests that the catalytic observations implicate telomerase as a critical link between cancer and aging. Although it is clear that subunit of TERT contributes to both normal and malignant cell physiology beyond its role in telomere maintenance. Specifically, expression of TERT drives tumor formation under situations when telomere length is not limiting and alters stem cell function in a manner that is independent of the expression of TERC. In prior work, we found that TERT is expressed in cycling primary pre-senescent human fibroblasts and that this low-level expression of telomerase controls proliferative lifespan. During the past funding period, we have investigated the role of TERT in both normal and malignant mammalian cells and found that in addition to its role in telomere maintenance, TERT regulates overall chromatin structure. Moreover, we and others have identified several novel protein complexes that associate with TERT. In recent work, we have found that TERT interacts with the RNA component of mitochondrial RNA processing endoribonuclease (RMRP), a gene that is mutated in the inherited pleiotropic syndrome Cartilage-Hair Hypoplasia. TERT and RMRP form an RNA dependent RNA polymerase (RdRP). These new observations suggest that TERT forms several distinct enzymatic complexes and provide a foundation to understand extra-telomeric functions of TERT. This proposal focuses on investigating the role of these newly identified TERT complexes in senescence and cancer. Specifically, biochemical, genetic and molecular biological approaches will be applied to investigate the role of the TERT-RMRP RdRP in chromatin homeostasis, senescence and transformation, to understand the roles of TERT-RMRP complexes in normal cells and tissues and to elucidate the function of TERT protein complexes in the regulation of senescence. These studies will not only provide new insights into the roles of various TERT complexes in aging and cancer but will also enhance our understanding of heterochromatin regulation. In addition, these studies will provide a foundation for strategies to manipulate these complexes therapeutically.